Copyright 2019 Discovery Owner's Association, Inc.

Fuel Filter Tip

If you have been around diesel motorhomes any length of time, you know that clean air and fuel are vital for optimal engine performance.

Changing an air filter is a relatively easy task, but replacement of both the primary and secondary fuel filter (if equipped) at each oil change interval can be a bit more challenging.

Typically, on the newer model Discovery, your primary fuel filter (10 micron), known as the fuel/water separator, is on the suction side of the fuel deliver system and is mounted to the chassis rail. The secondary filter (2 micron) is on the pressure side of the fuel delivery system mounted directly on the engine and arguably, plays the most important role of the two in preventing the introduction of contaminants into your engine.

Although the primary filter can be pre-filled prior to installation, Cummins recommends the secondary spin-on filter be installed dry to ensure that only adequately filtered fuel is used to fill the contents. This is accomplished by cycling the ignition switch on and off in 30 second intervals in order to activate the electronic fuel pump until the filter is full. Some Discovery owners find this procedure to be inadequate and as a result, have a hard time starting their coaches afterward due to excessive air in the fuel lines. In frustration, they disregard the preferred method, fill the secondary filter to the top, spin it on, and then hope for the best.

Unfortunately, this could be a costly mistake if the fuel you’re using hasn’t been filtered properly and makes its way into the fuel injection system. So, if you are one of those Discovery owners inclined to pre-fill your secondary filter prior to installation, I have a tip that just may save you a lot of money and aggravation.

Purchase a nylon threaded cap nut (see photo) at your local hardware or home improvement center and screw it into the threaded (center) hole of the filter. Then, fill the filter through the surrounding holes in the baffle or thread plate until the center tube is full. This cautionary approach will help prevent unfiltered fuel from entering the center tube prior to installation and should minimize the time and effort to get you up and running again.


The DISCOVERY OWNERS ASSOCIATION, INC, was formed to promote the sharing of information and the camaraderie of fellow Discovery motorhome owners. Membership in the club is limited to owners of Fleetwood Discovery motorhomes.


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